Image forming apparatus

ABSTRACT

An image forming apparatus may include a transfer nip, a first conveyance belt, a second conveyance belt, a fixing nip, a fixing drive unit, and a control unit. The first conveyance belt conveys the sheet to which a toner image is transferred and fixed. The fixing drive unit drives the fixing nip so that the fixing nip conveys the sheet at a speed that is higher than the transfer speed in a high speed fixing mode in which the fixing nip fixes the toner image to the sheet while conveying the sheet. The control unit sets a circumferential speed of the first conveyance belt and a circumferential speed of the second conveyance belt so that the sheet bridging the first and second conveyance belts is pulled by the second conveyance belt to slide on the first conveyance belt in the high speed fixing mode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus which formsan image on a sheet.

2. Description of the Related Art

Heretofore, there has been known an image forming apparatus in which atoner image formed on an image bearing member, such as a photosensitivemember or an intermediate transfer member, is transferred onto a sheetby a transfer unit and the toner image transferred onto the sheet isheated and fixed by a fixing unit.

The fixing unit applies heat which is suited for melting and fixing thetoner onto the sheet in the fixing unit. The quality of heat variesaccording to the kind of the sheet. According, an apparatus changes theconveyance speed of a sheet in a fixing unit according to the kind of asheet, which changes the quantity of heat applied to the sheet accordingto the kind of the sheet.

Japanese Patent Application Laid-Open No. 2006-251441 discusses an imageforming apparatus which makes faster the conveyance speed of a sheet ina fixing unit than in a transfer unit in a case where the sheet is plainpaper (whose thickness is normal). The image forming apparatus arrangestwo conveyance belts side by side in the conveyance direction betweenthe transfer and fixing units. The two conveyance belts convey the sheetwhile sucking the sheet to convey the sheet holding the toner yet to befixed to the fixing unit. Each length of the two conveyance belts in thedirection in which the sheet is conveyed is longer than the maximumlength of a usable sheet. In a configuration described in JapanesePatent Application Laid-Open No. 2006-251441, the sheet is conveyed asdescribed below.

The sheet sent out from the transfer unit is received by an upstreamconveyance belt driven at a conveyance speed in the transfer unit andthe upstream conveyance belt conveys the sheet to a downstreamconveyance belt similarly driven at the conveyance speed in the transferunit. Before the leading edge of the sheet reaches the fixing unit andwhen the trailing edge of the sheet reaches the downstream conveyancebelt, the conveyance speed of the downstream conveyance belt is changedto a fixing speed. The downstream conveyance belt driven at the fixingspeed conveys the sheet to the fixing unit.

More specifically, the length of the downstream conveyance belt in thesheet conveyance direction is longer than the maximum length of theusable sheet, which sometimes carries and conveys the sheet only by thedownstream conveyance belt, and, at this point, the conveyance speed ofthe downstream conveyance belt is changed to the fixing speed. In theabove image forming apparatus, however, when the trailing edge of thesheet to be conveyed reaches the downstream conveyance belt, theconveyance speed of the downstream conveyance belt is changed to a highfixing speed. In other words, the sheet is conveyed at a low transferspeed until the trailing edge of the sheet to be conveyed moves awayfrom the upstream conveyance belt. This delays the timing at which theconveyance speed of the sheet is changed to the high fixing speed. Thetiming at which the speed of the sheet advancing from the transfer unitis changed to the high speed is slow, taking the sheet advancing fromthe transfer unit longtime to reach the fixing unit, which lowersproductivity.

SUMMARY OF THE INVENTION

The present invention is directed to increasing productivity byshortening the time until a sheet advancing from a transfer unit reachesa fixing unit in an image forming apparatus capable of forming an imageon a sheet with a fixing speed made higher than a transfer speed.

According to an aspect of the present invention, an image formingapparatus includes a transfer nip configured to transfer a toner imageto a sheet conveyed at a transfer speed, a first conveyance beltconfigured to convey the sheet to which the toner image is transferredat the transfer nip, a second conveyance belt provided downstream of thefirst conveyance belt in a conveyance direction in which the sheet isconveyed and configured to convey the sheet, a fixing nip provideddownstream of the second conveyance belt in the conveyance direction andconfigured to fix the toner image to the sheet while conveying thesheet, a fixing drive unit configured to drive the fixing nip so thatthe fixing nip conveys the sheet at a speed that is higher than thetransfer speed in a high speed fixing mode in which the fixing nip fixesthe toner image to the sheet while conveying the sheet at the speed thatis higher than the transfer speed, and a control unit configured to seta circumferential speed of the first conveyance belt to a first speed atwhich the sheet advances from the transfer nip and a circumferentialspeed of the second conveyance belt to a second speed at which a leadingedge of the sheet enters the fixing nip and which is higher than thefirst speed so that the sheet bridging the first and second conveyancebelts is pulled by the second conveyance belt to slide on the firstconveyance belt in the high speed fixing mode.

According to an exemplary embodiment of the present invention,productivity can be increased in an image forming apparatus capable offorming an image on a sheet with a fixing speed made higher than atransfer speed.

Further features and aspects of the present invention will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments, features,and aspects of the invention and, together with the description, serveto explain the principles of the invention.

FIG. 1 illustrates a schematic diagram of an image forming apparatusaccording to a first exemplary embodiment.

FIG. 2 is an enlarged view between a secondary transfer unit and afixing unit in the first exemplary embodiment.

FIG. 3 is a top view of a second conveyance belt unit in the firstexemplary embodiment.

FIG. 4 is a table indicating a relationship among fixing speed, type,and grammage of a sheet.

FIG. 5 is a block diagram illustrating the control unit of the imageforming apparatus according to the first exemplary embodiment.

FIG. 6 is a flow chart illustrating the operation of the image formingapparatus according to the first exemplary embodiment.

FIG. 7 is a flow chart illustrating a large-size conveyance control inthe image forming apparatus according to the first exemplary embodiment.

FIGS. 8A to 8D illustrate schematic diagrams illustrating operation in acase where a large-size sheet is conveyed in a mode in which the fixingspeed is higher than the transfer speed in the first exemplaryembodiment.

FIGS. 9A to 9D illustrate schematic diagrams illustrating operation in acase where a small-size sheet is conveyed in a mode in which the fixingspeed is higher than the transfer speed in the first exemplaryembodiment.

FIGS. 10A and 10B illustrate schematic diagrams illustrating operationin a case where the large-size sheet is conveyed in a mode in which thefixing speed is equal to the transfer speed in the first exemplaryembodiment.

FIG. 11 is an enlarged view between a secondary transfer unit and thefixing unit in a second exemplary embodiment.

FIGS. 12A and 12B illustrate top views of a second conveyance belt unitin the second exemplary embodiment.

FIGS. 13A and 13B illustrate perspective views of the second conveyancebelt unit in the second exemplary embodiment.

FIG. 14 is a block diagram illustrating the control unit of the imageforming apparatus according to the second exemplary embodiment.

FIG. 15 is a flow chart illustrating the operation of the image formingapparatus according to the second exemplary embodiment.

FIG. 16 is a flow chart illustrating a large-size conveyance control inthe image forming apparatus according to the second exemplaryembodiment.

FIGS. 17A to 17C illustrate schematic diagrams illustrating operation ina case where the large-size sheet is conveyed in the mode in which thefixing speed is higher than the transfer speed in the second exemplaryembodiment.

FIGS. 18A and 18B illustrate schematic diagrams illustrating operationin a case where the large-size sheet is conveyed in the mode in whichthe fixing speed is higher than the transfer speed in the secondexemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the inventionwill be described in detail below with reference to the drawings.

FIG. 1 is a cross section illustrating a general configuration of animage forming apparatus according to a first exemplary embodiment.

An image forming apparatus 100 includes a feeding unit 110 for feeding astored sheet, an image forming unit 920 for forming a toner image on thesheet fed by the feeding unit 110, and a belt conveyance unit 904 forconveying the sheet on which the toner image is formed to a fixing unit50. The image forming apparatus 100 further includes a post conveyanceunit 903 for conveying the sheet to which the toner image is fixed bythe fixing unit 50.

The feeding unit 110 includes a sheet cassette 111 for storing sheets, apickup roller 112 for picking up the sheet from the sheet cassette 111,and a separation device 113 for separating and feeding the sheet pickedup by the pickup roller 112. The feeding unit 110 further includes afeeding roller 114 for conveying the sheet on a feeding path 901 alongwhich the sheet separated and fed by the separation device 113 isconveyed and a registration roller 115.

The image forming unit 920 is a tandem image forming unit in which imageforming stations 200Y, 200M, 200C, and 200K corresponding to yellow (Y),magenta (M), cyan (C), and black (K) respectively are arranged inseries. Each of the image forming stations 200Y, 200M, 200C, and 200Kincludes a photosensitive drum 120, a primary charge device 121, anexposure device 122, and a development device 123.

The image forming unit 920 includes an intermediate transfer belt 125 towhich the toner image visualized by the image forming stations 200Y,200M, 200C, and 200K is transferred. The intermediate transfer belt 125is stretched and supported by a drive roller 126, a tension roller 127,and a counter roller 128 and driven by the drive roller 126 to berotated in the direction of an arrow R2.

Pressing a secondary transfer roller 131 against the intermediatetransfer belt 125 supported by the counter roller 128 from the insideforms a secondary transfer nip N2 between the secondary transfer roller131 and the intermediate transfer belt 125. A secondary transfer unit130 is comprised of the secondary transfer roller 131, the intermediatetransfer belt 125, and the counter roller 128. A belt cleaning device129 rubs a cleaning web against the intermediate transfer belt 125 toremove a residual transfer toner and paper dust remaining on the surfaceof the intermediate transfer belt 125 passing through the secondarytransfer nip N2.

The fixing unit 50 arranged more downstream than the secondary transferunit 130 in the sheet conveyance direction fixes a toner image to thesheet by heat and pressure. The fixing unit 50 includes a heating roller52, which incorporate a heater therein, and a pressure roller 53. Thefixing unit 50 further includes a heating roller temperature sensor 70for detecting the surface temperature of the heating roller 52 and apressure roller temperature sensor 71 for detecting the surfacetemperature of the pressure roller 53. The heating roller temperaturesensor 70 and the pressure roller temperature sensor 71 are provided tokeep the surface temperature of the heating roller 52 and the pressureroller 53 at an appropriate level.

The belt conveyance unit 904 is arranged between the secondary transferunit 130 and the fixing unit 50. The belt conveyance unit 904 iscomposed of a first belt conveyance unit 10 arranged upstream in theconveyance direction and a second belt conveyance unit 20 arrangeddownstream. The configuration of the belt conveyance unit 904 isdescribed in detail below.

The post conveyance unit 903 includes a discharge roller 911 fordischarging the sheet discharged from the fixing unit 50 outside theapparatus. The post conveyance unit 903 further includes a reversingroller 912 for reversing and conveying the sheet and a two-sidedconveyance path 913 which conveys the sheet reversed by the reversingunit and is merged into the feeding path 901 on the downstream side.

Operation related to image formation in the image forming apparatus 100is roughly described below.

The exposure device 122 exposes the photosensitive drum 120 to form anelectrostatic latent image on the photosensitive drum 120. Theelectrostatic latent image on the photosensitive drum 120 is developedby the development device 123 and visualized as a toner image.

The toner image carried on the surface of the photosensitive drum 120 issequentially superimposed on the intermediate transfer belt 125, whichis referred to as a primary transfer. The toner image on theintermediate transfer belt 125 to which all colors of Y, M, C, and K areprimary-transferred is secondary-transferred to a sheet S fed by thefeeding unit 110 at the secondary transfer nip N2. The intermediatetransfer belt 125 is rotatably driven by the drive roller 126 rotatingat a constant speed and the circumferential speed thereof is kept at aconstant transfer speed. The sheet to which the toner image istransferred is conveyed by the secondary transfer nip N2 at the transferspeed. In the present exemplary embodiment, the transfer speed is set to300 mm/sec. The transfer speed refers to the speed of a sheet at thetime of transferring the toner image.

The registration roller 115 of the feeding unit 110 receives the sheet Sin a stop state, puts the sheet S on standby, and sends out the sheet Sto the secondary transfer nip N2 in timed relationship with the tonerimage on the intermediate transfer belt 125.

The sheet S carrying the toner image transferred at the secondarytransfer nip N2 is conveyed to the fixing unit 50 by the belt conveyanceunit 904. In the fixing unit 50, a fixing nip N nips the sheet S, andthe application of heat and pressure to the toner image yet to be fixedfixes the toner image to the sheet. The sheet subjected to the fixingprocess and sent out from the fixing unit 50 is discharged outside theapparatus by the discharge roller 911.

In a case where an image is formed on two sides, the sheet sent out fromthe fixing unit 50 is transferred to the reversing roller 912, reversedby the reversing roller 912, and then guided to the feeding path 901 viathe two-sided conveyance path 913. The toner image is formed on theother side of the sheet as is the case with the one side thereof.

The detailed configuration of the belt conveyance unit 904 and itsperiphery is described below using FIG. 2, which is an enlarged viewillustrating the secondary transfer unit 130, the belt conveyance unit904, and the fixing unit 50, and FIG. 3, which is a top view of the beltconveyance unit 904.

The belt conveyance unit 904 includes the first belt conveyance unit 10arranged more downstream than the secondary transfer nip N2 in the sheetconveyance direction and the second belt conveyance unit 20 arranged onthe downstream side of the first belt conveyance unit 10 and moreupstream than the fixing nip N.

A transfer guide 951 for guiding the sheet advancing from the secondarytransfer nip N2 to the belt conveyance unit 904 is provided between thebelt conveyance unit 904 and the secondary transfer nip N2. A pre-fixingguide 952 for guiding the sheet conveyed by the belt conveyance unit 904to the fixing nip N is provided between the belt conveyance unit 904 andthe fixing nip N.

The first belt conveyance unit 10 includes a first endless conveyancebelt 11 provided with a large number of holes, a first drive roller 12,and a driven roller 13, between which the first endless conveyance belt11 is stretched. The first belt conveyance unit 10 includes a firstconveyance belt drive motor 14 for rotating the first conveyance belt 11via the first drive roller 12. Inside the first conveyance belt 11 isarranged a first suction fan 15 as a first suction device which attractsthe sheet conveyed by the first conveyance belt 11 on thecircumferential surface of the first conveyance belt 11 via a largenumber of holes formed in the first conveyance belt 11. In other words,the first suction fan 15 sucks air from the upper surface side of thefirst conveyance belt 11 to attract the conveyed sheet on the uppersurface of the first conveyance belt 11. The first conveyance belt drivemotor 14 performs drive so that the circumferential speed V1 of thefirst conveyance belt 11 (hereinafter referred to as a conveyance speedV1 of the first belt conveyance unit 10) becomes equal to 300 mm/sec,which is the same speed as the transfer speed VT.

The second belt conveyance unit 20 is substantially similar inconfiguration to the first belt conveyance unit 10. The second beltconveyance unit 20 includes a second endless conveyance belt 21 providedwith a large number of holes as illustrated in FIG. 3, a second driveroller 22, and a driven roller 23, between which the second conveyancebelt 21 is stretched. The second belt conveyance unit 20 furtherincludes a second conveyance belt drive motor 24 for rotating the secondconveyance belt 21 via the second drive roller 22. Inside the secondconveyance belt 21 is arranged a second suction fan 25 as a secondsuction device which attracts the sheet conveyed by the secondconveyance belt 21 on the circumferential surface of the secondconveyance belt 21 via a large number of holes formed in the secondconveyance belt 21. In other words, the second suction fan 25 sucks airfrom the upper surface side of the second conveyance belt 21 to attractthe conveyed sheet on the upper surface of the second conveyance belt21. The second conveyance belt drive motor 24 can change thecircumferential speed V2 of the second conveyance belt 21 (hereinafterreferred to as a conveyance speed V2 of the second belt conveyance unit20).

On the side of the fixing unit 50 in the second conveyance belt 21 isprovided a sheet leading edge detection sensor 26 for detecting theleading edge of a sheet in a detection position P that is more upstreamthan the fixing nip N. The detection position P where the sheet leadingedge detection sensor 26 detects a sheet is set to such a position thatthe trailing edge of the sheet passes the secondary transfer nip N2 whenthe leading edge of the sheet reaches the detection position P. In thepresent exemplary embodiment, the distance between the secondarytransfer nip N2 and the detection position P is set longer than thelength L of the longest sheet S (hereinafter referred to as the lengthof the maximum sheet) in the conveyance direction which can be used bythe image forming apparatus. In the present exemplary embodiment, thedetection position P where the sheet leading edge detection sensor 26detects a sheet is set to the downstream end of the second conveyancebelt 21.

In the fixing unit 50, the heating roller 52 is rotatably driven by aheating roller drive motor 54 (a DC brushless motor, for example) as afixing drive unit. The circumferential speed of the heating roller 52(hereinafter referred to as a fixing speed VF) can be changed. Thefixing speed refers to the speed of a sheet at which the sheet to whichthe toner image is fixed by the fixing unit 50 is conveyed. The sheet Sconveyed by the fixing nip N formed between the heating roller 52 andthe pressure roller 53 is conveyed at the fixing speed VF.

The length of a sheet conveyance path between the secondary transfer nipN2 and the fixing nip N is set longer than the length L of the maximumsheet. The length LB of the sheet conveyance path between the sheetleading edge detection sensor 26 and the fixing nip N is set longer thana distance required for changing the speed of the second conveyance belt21.

The length B of the first and second conveyance belts 11 and 21 is equalto each other in the direction in which the sheet S is conveyed andsomewhat shorter than the half of the length L of the maximum sheet. Inthe present exemplary embodiment, the first and second conveyance belts11 and 21 are similar in configuration to use common components.However, the present invention is not limited to the first and secondconveyance belts 11 and 21 being equal to each other in length (beingsimilar in configuration). For example, the length of the firstconveyance belt 11 in the conveyance direction may be taken as ⅔ of thelength L of the maximum sheet and the length of the second conveyancebelt 21 in the conveyance direction may be taken as ⅓ of the length L ofthe maximum sheet.

FIG. 5 is a block diagram illustrating the control of the image formingapparatus 100. A control unit 170 is comprised of an arithmeticprocessing unit including a central processing unit (CPU) and a memoryand a circuit for transferring data with the outside such as aninput/output (I/O) port and a communication interface. The control unit170 controls the operation of the image forming apparatus bysequentially executing a plurality of programs stored in a memoryaccording to situations.

The control unit 170 controls each unit such as a feeding unit 110 andan image forming unit 920 according to a print job sent from an externaldevice. An operation unit connected to the control unit 170 sendsinformation about the type of a sheet (such as grammage, size, plainpaper or coated paper) to the control unit 170. The information sent asthe print job from the external device includes also the type of asheet. The coated paper refers to a sheet coated with resin. A signal isinput from the sheet leading edge detection sensor 26 to the controlunit 170. The control unit 170 controls the operation of the first andsecond conveyance belt drive motors 14 and 24, the first and secondsuction fans 15 and 25, and the heating roller drive motor 54.

Signals are input from the heating roller temperature sensor 70 and thepressure roller temperature sensor 71 in the fixing unit 50 to thecontrol unit 170. The control unit 170 also controls the temperature ofthe heating roller 52 and the pressure roller 53. The control unit 170controls the energization of a heater provided inside the heating roller52 via a temperature control unit based on the output of the heatingroller temperature sensor 70 so that the surface temperature of theheating roller 52 can be kept at a preset temperature. The control unit170 controls the energization of a heater provided inside the pressureroller 53 via the temperature control unit based on the output of thepressure roller temperature sensor 71 so that the surface temperature ofthe pressure roller 53 can be kept at a preset temperature. In thepresent exemplary embodiment, the control unit 170 controls temperatureso that the surface temperature of the heating roller 52 can be kept at170° C. and the surface temperature of the pressure roller 53 can bekept at 100° C.

The control unit 170 controls the operation of the heating roller drivemotor 54 in the fixing unit 50 so that the fixing speed is variedaccording to the grammage and type (plain paper or coated paper) amongthe information about the type of a sheet. FIG. 4 is a table indicatingcorrespondence among the type of the sheet S, the grammage thereof, andthe set fixing speed VF. Quantity of heat with which the fixing unit 50supplies the sheet per unit area of the sheet is greater in a mode 2than in a mode 1. Quantity of heat with which the fixing unit 50supplies the sheet per unit area of the sheet is greater in a mode 3than in the mode 2. Quantity of heat with which the fixing unit 50supplies the sheet per unit area of the sheet is greater in a mode 4than in the mode 3.

As illustrated in FIG. 4, the fixing speed for plain paper with agrammage of approximately 60 gsm (grams per square meter) is set to 480mm/sec, for example. The quantity of heat of plain paper with a grammageof approximately 60 gsm is smaller than that of plain paper with agrammage of approximately 200 gsm, for example. For this reason, in acase where an image is fixed to plain paper with a grammage ofapproximately 60 gsm, the fixing speed is set to 480 mm/sec which isfaster than 300 mm/sec to which the fixing speed is set in a case wherean image is fixed to plain paper with a grammage of approximately 200gsm, thereby reducing time for which the sheet S passes the fixing nipN. This makes smaller the quantity of heat with which the fixing unit 50supplies the sheet per unit area in a case where an image is fixed toplain paper with a grammage of approximately 60 gsm than in a case wherean image is fixed to plain paper with a grammage of approximately 200gsm.

The greater the grammage of the sheet, the greater the quantity of heat.Therefore, the greater the grammage of the sheet, the greater thequantity of heat required per unit area of the sheet for heating thesheet to the temperature at which the toner is fixed to the sheet. Asillustrated in FIG. 4, the fixing speed is lowered according as thegrammage increases to increase the quantity of heat supplied per unitarea of the sheet. Greater quantity of heat is required for fixing animage per unit area of the sheet in coated paper than in plain paper, sothat the fixing speed is varied depending on the plain paper or thecoated paper, as illustrated in the table of FIG. 4.

The control unit 170 adjusts also the conveyance speed V2 of the secondbelt conveyance unit 20 within the range of 300 mm/sec to 480 mm/secaccording to speed adjustment of the fixing speed VF. The conveyancespeed V2 of the second belt conveyance unit 20 is adjusted by thecontrol unit 170 controlling the second conveyance belt drive motor 24.

The control and operation of conveyance of the sheet from the secondarytransfer nip N2 to the fixing unit 50 are described below with referenceto flow charts in FIGS. 6 and 7 and operation charts in FIGS. 8A to 8Dto FIGS. 10A and 10B illustrating the state of conveyance of the sheet.

In the flow chart in FIG. 6, the control unit 170 determines which themode is in FIG. 4 based on the type and the grammage among theinformation about the sheet input from the operation unit. In step S1,the control unit 170 determines whether the transfer speed and thefixing speed are in the same mode (mode 4) with reference to the tablein FIG. 4.

If the mode is not the mode 4 (the fixing speed is faster than thetransfer speed) (NO in step S1), then in step S2, the control unit 170determines whether the sheet is large or small in size based on sizeinformation among the information about the type of the sheet. In thepresent exemplary embodiment, the control unit 170 determines that thesheet is small in size in a case where the length of the sheet to beconveyed in the conveyance direction is shorter than the length from thesecondary transfer nip N2 to the upstream end of the secondary transferbelt 21 and shorter than the length from the downstream end of the firsttransfer belt 11 to the fixing nip N. In step S2, if the control unit170 determines that the sheet is not small in size, the control unit 170executes a large-size conveyance control illustrated in FIG. 7. FIGS. 8Ato 8D illustrate time-series charts illustrating the operation ofconveyance of the sheet in performing the control in the mode in whichthe fixing speed is faster than the transfer speed and the size in whichthe size of the sheet is large (hereinafter referred to as a large-sizeconveyance control).

In the large-size conveyance control, in step S21, the control unit 170controls the heating roller drive motor 54 to keep the fixing speed VFillustrated in FIG. 4 according to the type and the grammage of thesheet. The control unit 170 stops (OFF) the operation of the firstsuction fan 15 and rotates (ON) the second suction fan 25. The controlunit 170 controls the first conveyance belt drive motor 14 so that theconveyance speed V1 of the first belt conveyance unit 10 becomes equalto 300 mm/sec which is equal to the transfer speed and is a first speedat which the sheet advancing from the transfer nip N2 is received. Instep S21, the control unit 170 controls the second conveyance belt drivemotor 24 so that the conveyance speed V2 of the second belt conveyanceunit 20 becomes equal to 300 mm/sec which is equal to the transfer speedand is a first speed at which the sheet advancing from the transfer nipN2 is received.

It is exemplified herein that the first speed of the first and secondbelt conveyance units 10 and 20 at which the sheet advancing from thetransfer nip N2 is received is equal to the transfer speed. However, thefirst speed of the first and second belt conveyance units 10 and 20 hasonly to be a speed at which the sheet advancing from the transfer nip N2can be smoothly conveyed without significantly affecting the transfer ofthe toner image and does not always need to be equal to the transferspeed. For example, the first speed at which the sheet advancing fromthe transfer nip N2 is received may be speed which is faster by 4% thanthe transfer speed. This is because the sheet is conveyed with the sheetnipped with the transfer nip N2 to allow the sheet to be conveyed whilesliding on the first and second conveyance belts 11 and 12. The firstspeed may be set to a speed which is somewhat slower than the transferspeed and little affects the transfer.

In such an operation state, the sheet sent out from the transfer nip N2is conveyed by the first belt conveyance unit 10. The sheet is sent tothe second belt conveyance unit 20 by the first belt conveyance unit 10and the leading edge side of the sheet is conveyed by the second beltconveyance unit 20. At this point, the circumferential speed of thefirst conveyance belt 11 (the conveyance speed V1 of the first beltconveyance unit 10) is set to the first speed at which the sheetadvancing from the transfer nip N2 is received (in the present exemplaryembodiment, the first speed is equal to 300 mm/sec being the transferspeed). The circumferential speed of the second conveyance belt 21 (theconveyance speed V2 of the second belt conveyance unit 20) is set alsoto the first speed (300 mm/sec) at which the sheet advancing from thetransfer nip N2 is received. Therefore, as illustrated in FIG. 8A,although the sheet S1 lies on the first and second conveyance belts 11and 12 and the trailing edge of the sheet S1 is still nipped by thesecondary transfer nip N2, the sheet is smoothly conveyed. The smoothconveyance of the sheet satisfactorily transfers the toner image in thesecondary transfer nip N2.

In step S22, the control unit 170 determines whether the leading edge ofthe sheet conveyed by the first and second conveyance belts 11 and 12 isdetected by the sheet leading edge detection sensor 26. FIG. 8Billustrates a state at the time when the leading edge of the sheetconveyed by the first and second conveyance belts 11 and 12 reaches thesheet leading edge detection sensor 26.

If the leading edge of the sheet conveyed by the first and secondconveyance belts 11 and 12 reaches a detection position P (YES in stepS22), then in step S23, the control unit 170 controls the secondconveyance belt drive motor 24 so that the conveyance speed V2 of thesecond belt conveyance unit 20 is changed from 300 mm/sec as the firstspeed to the fixing speed VF as a second speed at which the leading edgeof the sheet enters the fixing nip N. As described above, the fixingspeed VF is set according to the type and grammage of the sheet withreference to the table in FIG. 4.

It is exemplified herein that the second speed of the second beltconveyance unit 20 at which the leading edge of the sheet enters thefixing nip N is equal to the fixing speed. However, the second speed ofthe second belt conveyance unit 20 has only to be a speed at which thesheet can be conveyed such that the toner image can be satisfactorilyfixed when the leading edge of the sheet enters the fixing nip N anddoes not always need to be equal to the fixing speed. For example, thesecond speed at which the leading edge of the sheet enters the fixingnip N may be lower by several percent or higher by several percent thanthe fixing speed. If the second speed of the second conveyance belt 21is made higher by several percent than the fixing speed, the sheet isbent between the second conveyance belt 21 and the fixing nip Naccording as the sheet is conveyed after the leading edge of the sheetenters the fixing nip N. If the second speed of the second conveyancebelt 21 is made lower by several percent than the fixing speed, thesheet is conveyed while sliding on the second conveyance belt 21 afterthe leading edge of the sheet enters the fixing nip N.

As described above, the detection position P where the sheet leadingedge detection sensor 26 detects the leading edge of the sheet is set tosuch a position that the trailing edge of the sheet passes the secondarytransfer nip N2 when the leading edge of the sheet reaches the detectionposition P. For this reason, the trailing edge of the sheet alreadypasses the secondary transfer nip N2 when the conveyance speed of thesecond belt conveyance unit 20 is increased from 300 mm/sec to thesecond speed (the fixing speed VF in the present exemplary embodiment).Accordingly, the increase of the conveyance speed of the second beltconveyance unit 20 from the first speed to the second speed does notaffect the transfer of the toner image to the sheet. The detectionposition P is set to such a position that the second belt conveyanceunit 20 can complete the increase of the conveyance speed of the secondbelt conveyance unit 20 from the first speed to the second speed untilthe leading edge of the sheet S1 reaches the fixing nip N of the fixingunit 50.

In the first exemplary embodiment, the following sheet conveyance isperformed by stopping the first suction fan 15 and operating the secondsuction fan 25. More specifically, the stoppage of the first suction fan15 does not cause the circumferential surface of the first conveyancebelt 11 to suck the sheet and the operation of the second suction fan 25causes the circumferential surface of the second conveyance belt 21 tosuck the sheet. As a result, the conveyance force of the secondconveyance belt 21 is greater than that of the first conveyance belt 11.Therefore, the sheet is conveyed at the second speed while sliding onthe first conveyance belt 11 by the second conveyance belt 21 whosespeed is changed to the second speed higher than the circumferentialspeed of the first conveyance belt 11 and the leading edge of the sheetS1 reaches the fixing nip N.

While the sheet is being conveyed while sliding on the first conveyancebelt 11 by the second conveyance belt 21, a succeeding sheet S2 conveyedby the secondary transfer nip N2 is conveyed by the first conveyancebelt 11 as illustrated in FIG. 8C. Setting the conveyance speed of thefirst belt conveyance unit 10 to the first speed (transfer speed) stablyconveys the succeeding sheet S2. In other words, the circumferentialspeed of the first conveyance belt 11 is kept at the first speed(transfer speed) to allow the conveyance of continuous sheets with thespace interval therebetween shortened, increasing the productivity ofimage formation. Thus, the leading edge of the succeeding sheet S2 iscaused to lie on the first conveyance belt 11 while a stable conveyanceis being realized, with the fixing nip N fixing the toner image to thepreceding sheet S1, whose trailing edge lies on the first conveyancebelt 11. This allows shortening the space interval between the trailingedge of the preceding sheet S1 and the leading edge of the succeedingsheet S2 to increase productivity.

When the leading edge of the sheet S1 conveyed by the second beltconveyance unit 20 reaches the fixing nip N, the sheet S1 conveyed atthe second speed (the fixing speed VF, in the present exemplaryembodiment) at which the leading edge of the sheet S1 enters the fixingnip N. A great difference between the conveyance speed of the secondbelt conveyance unit 20 and the fixing speed VF at the fixing nip N atthe time when the leading edge of the sheet S1 enters the fixing nip Ncauses a defective fixing. Conveying the sheet while sliding on thefirst conveyance belt 11 by the second c belt conveyance unit 20 at thesecond speed (the fixing speed VF) at which the leading edge of thesheet S1 enters the fixing nip N does not cause defects in fixing thetoner image.

After the control unit 170 determines that the leading edge of the sheetS1 reaches the fixing nip N (YES in step S24), then in step S25, thecontrol unit 170 controls the second conveyance belt drive motor 24 sothat the conveyance speed of the second belt conveyance unit 20 ischanged from the fixing speed VF being the second speed to 300 mm/secbeing the first speed. As described below, the control unit 170determines whether the leading edge of the sheet S1 reaches the fixingnip N. The control unit 170 receives the signal indicating that theleading edge of the sheet S1 is detected from the sheet leading edgedetection sensor 26. The timer 666 measures whether a predetermined timeinterval required by the leading edge of the sheet S1 for reaching thefixing nip N passes after the control unit 170 receives the signal fromthe sheet leading edge detection sensor 26, and the control unit 170determines whether the leading edge of the sheet S1 reaches the fixingnip N.

The reason the conveyance speed of the second belt conveyance unit 20 isreduced to the transfer speed being the first speed after the leadingedge of the sheet S1 is nipped by the fixing nip N is that an auxiliarytransfer is sufficient for the function required by the second beltconveyance unit 20 after the leading edge of the sheet S1 is nipped bythe fixing nip N. After the conveyance speed of the second beltconveyance unit 20 is reduced to the transfer speed, the sheet conveyedby the fixing nip N is conveyed while sliding the second conveyance belt21. This is because a high nipping pressure of the fixing nip N makesthe conveyance force of the fixing unit 50 stronger than that of thesecond conveyance belt 21.

The reason the conveyance speed of the second belt conveyance unit 20 isreduced to the transfer speed being the first speed in a state where theleading edge of the sheet S1 is nipped by the fixing nip N and thetrailing edge of the sheet S1 lies on the second conveyance belt 21 isdescribed below. The reason is that the leading edge of the succeedingsheet S2 is allowed to reach the second conveyance belt 21 while thepreceding sheet S1 is conveyed by the fixing nip N. This allowsproviding the image forming apparatus high in productivity becausecontinuous sheets can be conveyed with the space interval therebetweenshortened. While the trailing edge of the succeeding sheet S2 lies atthe secondary transfer nip N2, both of the circumferential speed of thefirst and second conveyance belts 11 and 21 are equal to the first speedat which the sheet advancing from the transfer nip N2 is received, sothat the succeeding sheet S2 can be stably conveyed to prevent adefective transfer from being produced.

The control unit 170 reduces the conveyance speed of the second beltconveyance unit 20 to 300 mm/sec being the first speed in step S25 andthen, in step S26, the control unit 170 determines whether the job isended. If the job is not ended (NO in step S26), the processing returnsto step S22. If the job is ended (YES in step S26), the processingreturns to step S3 in FIG. 6 and ends.

If the mode is the mode 4 in step S1, in other words, if the controlunit 170 determines that the fixing speed VF is equal to the transferspeed VT, the processing proceeds to step S4. In step S4, the controlunit 170 operates (ON) the first suction and second suction fans 15 and25. The control unit 170 controls the first conveyance belt drive motor14 so that the conveyance speed of the first belt conveyance unit 10 ischanged to the first speed (300 mm/sec being the transfer speed). Thecontrol unit 170 further controls the second conveyance belt drive motor24 so that the conveyance speed of the second belt conveyance unit 20 ischanged to the first speed (300 mm/sec being the transfer speed).Continuous sheets are conveyed as it is. FIGS. 10A and 10B illustratetime-series charts in a case where the fixing speed is equal to thetransfer speed and a large-size sheet is conveyed.

If the mode is not the mode 4 in step S1 in FIG. 6 (in a case where thefixing speed is higher than the transfer speed) and the sheet is small(YES in step S2), the processing proceeds to step S5. In step S5, thecontrol unit 170 operates (ON) the first suction and second suction fans15 and 25. The control unit 170 controls the first conveyance belt drivemotor 14 so that the conveyance speed of the first belt conveyance unit10 is changed to the first speed (300 mm/sec being the transfer speed).The control unit 170 further controls the second conveyance belt drivemotor 24 so that the conveyance speed of the second belt conveyance unit20 is changed to the second speed (equal to the fixing speed VF).Continuous sheets are conveyed as it is.

FIGS. 9A to 9D illustrate time-series charts in a case where asmall-size sheet is continuously conveyed at the fixing speed higherthan the transfer speed. The small-size refers to a size in which thetrailing edge of a sheet passes the secondary transfer nip N2 when theleading edge thereof reaches the second conveyance belt 21, and thetrailing edge of the sheet passes the first conveyance belt 11 when theleading edge thereof reaches the fixing nip N. FIGS. 9A to 9D illustratea conveyance state in a case where the transfer speed is faster than thefixing speed and the sheet to be conveyed is of small-sized.

FIG. 9A illustrates that the sheet to which the toner image istransferred at the transfer nip N2 is conveyed by the first conveyancebelt 11 whose circumferential speed is set to the transfer speed VT andwhich is performing a suction operation.

As illustrated in FIG. 9B, before the leading edge of the sheet carriedby the first conveyance belt 11 reaches the second conveyance belt 21,the trailing edge of the sheet leaves the transfer nip N2. When theleading edge of the sheet contacts the second conveyance belt 21 whichis rotated at the second speed higher than the transfer speed andperforms a suction operation, the transfer of the toner image to thesheet at the secondary transfer nip N2 is already completed. Therefore,the rotation of the second conveyance belt 21 at the second speed higherthan the transfer speed as it is does not affect the transfer of thetoner image by the second conveyance belt 21.

FIG. 9C illustrates a transient state in which the leading edge of thesheet reaches the second conveyance belt 21 and the sheet bridges thefirst and second conveyance belts 11 and 21. In an initial state wherethe major portion of the sheet lies on the first conveyance belt 11 anda part of the leading edge of the sheet contacts the second conveyancebelt 21, the sheet is conveyed as described below. The sheet is conveyedat the first speed by the first conveyance belt 11 while being sucked bythe first conveyance belt 11 and the part of the leading edge of thesheet slides on the second conveyance belt 21 rotated at the high secondspeed. Thereafter, the sheet advances and is sucked by the secondconveyance belt 21 according as the area is increased where the sheetcontacts the second conveyance belt 21. The sheet is pulled and conveyedby the second conveyance belt 21 while sliding on the first conveyancebelt 11. The sheet is conveyed at the high speed by the secondconveyance belt 21 before the trailing edge of the sheet passes thedownstream end of the first conveyance belt 11, so that the time periodfor which the sheet advancing from the transfer nip N2 reaches thefixing nip N is short, which increases productivity.

Thereafter, as illustrated in FIG. 9D, the leading edge of the sheetreaches the fixing nip N after the trailing edge of the sheet passes thefirst conveyance belt 11. The trailing edge of the sheet passes thedownstream end of the first conveyance belt 11 when the leading edge ofthe sheet reaches the fixing nip N, so that the sheet is conveyed at thesecond speed by the second conveyance belt 21 without being affected bythe first conveyance belt 11.

In the present exemplary embodiment, in the large-size conveyancecontrol, the control unit 170 stops (OFF) the rotation of the firstsuction fan 15 of the first belt conveyance unit 10 and operates (ON)the second suction fan 25 of the second belt conveyance unit 20.Thereby, the second conveyance belt 21 becomes greater in the force ofconveying the sheet S than the first conveyance belt 11 to allow thesheet to be conveyed while being pulled by the second conveyance belt 21and sliding on the first conveyance belt 11. Alternatively, for example,a difference in a suction force may be provided between the first andsecond suction fans 15 and 25 by half rotating the first suction fan 15and fully rotating the second suction fan 25 to make the secondconveyance belt 21 greater in the conveyance force than the firstconveyance belt 11, thereby sliding the sheet on the first conveyancebelt 11. Still alternatively, a shutter may be provided between thefirst suction fan 15 and the first conveyance belt 11 instead of aconfiguration in which the rotation of the first suction fan 15 isstopped. More specifically, air current of the first suction fan 15 isshut off by the shutter to reduce the suction force of the firstconveyance belt 11, decreasing the conveyance force of the firstconveyance belt 11.

Alternatively, in the large-size conveyance control, with the secondsuction fan 25 kept operated, the second conveyance belt 21 can be madegreater in the force of conveying the sheet than the first conveyancebelt 11 to cause the sheet to slide on the first conveyance belt 11. Forexample, their respective conveyance forces may be adjusted in designingby adjusting the number of holes and the size of the hole formed in thefirst and second conveyance belts 11 and 21, i.e., aperture ratio. Morespecifically, the aperture ratio of the second conveyance belt 21 ismade greater than that of the first conveyance belt 11 to provide adifference between a force of the first suction fan 15 sucking the sheetagainst the first conveyance belt 11 and a force of the second suctionfan 25 sucking the sheet against the second conveyance belt 21. Thereby,the second conveyance belt 21 is made greater in the force of conveyingthe sheet than the first conveyance belt 11 to cause the sheet to slideon the first conveyance belt 11. The frictional coefficient of theconveyance surface of the first conveyance belt 11 is made smaller thanthat of the second conveyance belt 21 to make a sheet conveying force ofthe second conveyance belt 21 sucking the sheet greater than that of thefirst conveyance belt 11 sucking the sheet, thereby, the sheet may beconveyed while being pulled by the second conveyance belt 21 and slidingon the first conveyance belt 11. For example, the surface of theconveyance belt on which the sheet is conveyed is coated with Teflon(registered trademark) to allow decreasing the frictional coefficient ofthe sheet conveyance surface to 0.3 to 0.4 from 1.0 to 1.2. The surfaceof the first conveyance belt 11 on which the sheet is conveyed is coatedwith Teflon (registered trademark) and the second conveyance belt ismade of rubber high in frictional coefficient, for example. Thus, thefrictional coefficient of the surface of the first conveyance belt 11 onwhich the sheet is conveyed is made smaller than that of the surface ofthe second conveyance belt 21 on which the sheet is conveyed. Such aconfiguration makes a sheet conveying force of the second conveyancebelt 21 sucking the sheet greater than that of the first conveyance belt11 sucking the sheet, which enables the second conveyance belt 21 tosurely pull the sheet and causes the sheet to slide on the firstconveyance belt 11.

As described above, the aperture ratio of the first and secondconveyance belts 11 and 21 is adjusted. The frictional coefficient ofthe first conveyance belt 11 is made smaller than that of the secondconveyance belt 21. Such a configuration is advantageous in conveyingthe small-sized sheet in a mode in which the fixing speed is higher thanthe transfer speed. The speed of the sheet depends on the secondconveyance belt 21 high in speed at the early time of the transientstate in which the sheet is conveyed with the sheet bridging the firstand second conveyance belts 11 and 21 (refer to FIG. 9C). This isbecause the sheet from the transfer nip N2 reaches the fixing nip Nearlier. In a case where the small-sized sheet is conveyed in a mode inwhich the fixing speed is higher than the transfer speed, the sheetconveyance force of the first conveyance belt 11 may be reduced bylowering the suction of the first suction fan 15 in the first beltconveyance unit 10. Also in this control, the speed of the sheet dependson the second conveyance belt 21 high in speed at the early time of thetransient state (refer to FIG. 9C).

In the present exemplary embodiment, in the large-size conveyancecontrol, the control unit 170 determines the timing at which theconveyance speed of the second belt conveyance unit 20 is changed fromthe first speed (the transfer speed) to the second speed (the fixingspeed VF) depending on whether the leading edge of the sheet is detectedby the sheet leading edge detection sensor 26. However, the timing atwhich the conveyance speed of the second belt conveyance unit 20 is madefaster from the first speed to the second speed has only to be performedafter the trailing edge of the sheet to be conveyed passes the secondarytransfer nip N2, so that the present invention is not always limited tothe above configuration. For example, a sensor for detecting the sheetis provided in the vicinity of the registration roller 115. A signal isreceived from the sensor and then the timer 666 measures whether thetime required for the trailing edge of the sheet passing the secondarytransfer nip N2 elapses. Thus, the control unit 170 determines that thetrailing edge of the sheet passes the secondary transfer nip N2.Alternatively, a sensor for detecting that the trailing edge of thesheet passes the transfer nip N2 may be provided between the transfernip N2 and the first conveyance belt 11 and the conveyance speed of thesecond belt conveyance unit 20 may be changed based on the signal of thesensor.

The first exemplary embodiment described above achieves the followingeffects.

(1) The fixing speed VF is changed according to the type and grammage ofthe sheet S. For this reason, the fixing unit can provide the sheet withan appropriate quantity of heat according to the type and grammage ofthe sheet S with the temperature of the fixing unit kept at a constantregulated temperature.

(2) The first and second belt conveyance units 10 and 20 arranged in theconveyance direction are provided between the secondary transfer nip N2and the fixing nip N. The conveyance speed V1 of the first beltconveyance unit 10 is taken as the first speed at which the sheetadvancing from the transfer nip N2 is received. The conveyance speed V2of the second belt conveyance unit 20 can be changed between the firstspeed and the second speed which is higher than the first speed and atwhich the leading edge of the sheet enters the fixing nip N. Therefore,the sheet S can be smoothly conveyed from the secondary transfer nip N2to the fixing nip N. Switching the conveyance speed of the sheet Sbetween the secondary transfer nip N2 and the fixing nip N allows thetransfer speed VT to be kept constant irrespective of the setting valueof the fixing speed VF.

(3) Even if the large-sized sheet is conveyed in a mode in which thefixing speed VF is higher than the transfer speed, the sheet can bestably conveyed with a high productivity using the conveyance belt shortin length in the conveyance direction.

The sheet is conveyed at the high second speed while sliding on thefirst conveyance belt 11 whose circumferential speed is the first speedby the second conveyance belt 21 whose speed is set to the second speedat which the leading edge of the sheet enters the fixing nip and, inthis state, the leading edge of the sheet is caused to reach the fixingnip N. The second conveyance belt 21 is minimized so that the length ofthe conveyance path from the downstream end of the first conveyance belt11 to the fixing nip N can be made smaller than that of the sheet to beconveyed and the speed of the sheet can be increased. The secondconveyance belt 21 is minimized and the speed of the sheet is changed tothe high second speed at the time early before the trailing edge of thesheet leaves the first conveyance belt 11, so that the sheet advancingfrom the secondary transfer nip N2 reaches early the fixing nip N. Thus,the length between the secondary transfer nip N2 and the fixing nip Ncan be shortened, the image forming apparatus can be minimized, andproductivity is high.

The succeeding sheet sent from the secondary transfer nip N2 can bereceived by the first conveyance belt 11 rotated at the first speedwhile the preceding sheet bridging the first and second conveyance belts11 and 21 is being conveyed at the second speed (the fixing speed) bythe second conveyance belt 21. A short interval between the continuouslyconveyed sheets with the toner image being satisfactorily transferredincreases productivity.

In order that the image forming apparatus is downsized by reducing thelength of the conveyance path between the transfer nip N2 and theupstream end of the second conveyance belt 21, in the case of thelarge-size sheet, the circumferential speed of the second conveyancebelt 21 is set to the first speed until the trailing edge of the sheetpasses the transfer nip N2 and then the circumferential speed of thesecond conveyance belt 21 is increased. Thus, the length of theconveyance path between the transfer nip N2 and the upstream end of thesecond conveyance belt 21 is reduced and the speed of the sheet isincreased before the trailing edge of the sheet leaves the downstreamend of the first conveyance belt 11 so that the time required for sheetreaching the fixing nip N from the transfer nip N2 is short.

Thus, the length between the secondary transfer nip N2 and the fixingnip N is reduced and the fixing speed VF is made higher than thetransfer speed VT to allow productivity to be increased and the sheet tobe smoothly conveyed.

(4) In the case of the small-size sheet which is shorter than the lengthbetween the transfer nip N2 and the upstream end of the secondconveyance belt 21 and shorter than the length between the downstreamend of the first conveyance belt 11 and the fixing nip N, the followingcontrol is performed even in a mode in which the fixing speed is higherthan the transfer speed. The sheet is conveyed without thecircumferential speed of the first and second conveyance belts and thestate of suction thereof being changed during the conveyance of thesheet. Therefore, in this case, a complicated control is not required inthe conveyance process, the speed of the sheet is changed to a highspeed at an early timing and productivity is high.

The distance between the secondary transfer nip N2 and the transfer nipN is longer than the length of the maximum-sized conveyable sheet. Asillustrated in FIG. 9B, for example, the leading edge of the sheetcarrying the toner image yet to be fixed reaches the fixing nip N afterthe trailing edge of the sheet S1 passes the secondary transfer nip N2.For this reason, even if shock is caused when thick paper plunges intothe fixing nip N, image defectiveness can be minimized because thetailing edge of the sheet S1 passes the secondary transfer nip N2.

In the first exemplary embodiment, in the large-size conveyance control,the conveyance speed of the second belt conveyance unit 20 is changedwhile the sheet is being conveyed, and in the large-size conveyancecontrol, the suction operation of the first and second suction fans 15and 25 is not changed while the sheet is being conveyed. The secondexemplary embodiment is different from the first exemplary embodiment inthat, in the large-size conveyance control, the suction operation of thefirst and second belt conveyance unit 10 and 20 is changed without theconveyance speed of the second belt conveyance unit 20 being changedwhile the sheet is being conveyed. The detailed description of theconfiguration of the second exemplary embodiment similar to thedescription of the first exemplary embodiment is omitted and only thepoints different from the first exemplary embodiment are described indetail below.

FIG. 11 is an enlarged view between a secondary transfer unit 131 andthe fixing unit 50 in the image forming apparatus of the secondexemplary embodiment. As illustrated in FIG. 11, an upstream sheetleading edge detection sensor 16 for detecting the leading edge of thesheet at an upstream detection position P1 is provided on the side ofthe fixing unit 50 in the first conveyance belt 11.

A configuration for sucking the sheet against the conveyance belt in thefirst and second belt conveyance units 10 and 20 is described in detailbelow with reference to FIGS. 12A and 12B and FIGS. 13A and 13B. Thefirst and second belt conveyance units 10 and 20 are similar to eachother in configuration, so that the second belt conveyance unit 20 isdescribed herein and the detailed description of the first beltconveyance unit 10 is omitted.

FIGS. 13A and 13B are perspective views of the second belt conveyanceunit 20 with the drawing of the second conveyance belt 21 omitted forthe sake of better comprehension of the configuration. Inside the secondconveyance belt 21 is provided a frame 951 incorporating the secondsuction fan 25. On the upper surface of the frame 951 is formed a pairof suction ports 25 a. A pair of second shutters 27 is slidably held bythe frame 951 between the second conveyance belt 21 and the secondsuction fan 25. The second shutter 27 is held by the frame 951 so thatthe second shutter 27 can slide between a position where the suctionport 25 a is opened (refer to FIG. 12A and FIG. 13A) and a positionwhere the suction port 25 a is closed (refer to FIG. 12B and FIG. 13B).

Slidably moving the second shutter 27 comprising the second suctiondevice along with the second suction fan 25 and the frame 951 allowschanging the state of sucking the sheet against the second conveyancebelt 21 in the second belt conveyance unit 20. Hereinafter, the positionof the second shutter 27 where the suction port 25 a is opened (refer toFIG. 12A and FIG. 13A) is referred to as an open position and theposition of the second shutter 27 where the suction port 25 a is closed(refer to FIG. 12B and FIG. 13B) is referred to as a closed position. Ina case where the second shutter 27 is in the open position illustratedin FIGS. 12A and 13A, the sheet is sucked against the second conveyancebelt 21. In a case where the second shutter 27 is in the closed positionillustrated in FIGS. 12B and 13B, the sheet is not sucked against thesecond conveyance belt 21.

The second shutters 27 are provided with rack gears. The transmission ofthe driving force of a second shutter drive motor 30 to the secondshutter 27 via a pinion gear engaging with the rack gear slides thesecond shutter 27 in the width direction to cause the suction port 25 aof the second suction fan 25 to open or close.

The second shutters 27 are provided with flags 27 b. The second shutter27 is moved to the open position illustrated in FIGS. 12A and 13A asdescribed below. The second shutter drive motor 30 is driven. When theflag 27 b interrupts the optical axis of a second shutter detectionsensor 29 to turn the signal from the second shutter detection sensor 29ON from OFF, the second shutter drive motor 30 is stopped. On the otherhand, in a case where the second shutter 27 is moved to the closedposition as illustrated in FIGS. 12B and 13B, the second shutter drivemotor 30 is reversely rotated by the predetermined number of pulses.

The first belt conveyance unit 10 is also similar to the second beltconveyance unit 20 in the shutter and the mechanism for moving theshutter. More specifically, a first shutter composing a first suctiondevice along with a first suction fan 15 is provided between the firstsuction fan 15 and the first conveyance belt 11 inside the firstconveyance belt 11. The first shutter is slidably moved by a firstshutter drive motor.

In the second exemplary embodiment illustrated in FIG. 14, signals areinput to a control unit 1170 from an upstream leading edge detectionsensor 16 and a sheet leading edge detection sensor 26. The control unit1170 controls the operation of the first and second conveyance beltdrive motors 14 and 24 and the heating roller drive motor 54. Signalsare input to the control unit 1170 from a first shutter detection sensor19 and a second shutter detection sensor 29. The control unit 1170operates the first and second shutter drive motors 31 and 30 based onthe signals from the first and second shutter detection sensors 19 and29 to move the shutters between the open and the closed positions. Thecontrol unit 1170 controls the suction operation of a suction unitcomprised of the first suction device including the first suction fan15, the first shutter drive motor 31, and a first shutter 17 and thesecond suction device including the second suction fan 25, the secondshutter drive motor 30, and the second shutter 27.

The control unit 1170 changes also the second speed being the conveyancespeed of the second belt conveyance unit 20 within the range of 300mm/sec to 480 mm/sec according to the speed adjustment of the fixingspeed VF. The conveyance speed of the second belt conveyance unit 20 ischanged by the control unit 1170 controlling a second conveyance beltdrive motor 24.

The operation and control of conveyance of the sheet from the secondarytransfer nip N2 to the fixing unit 50 are described in detail below withreference to flow charts in FIGS. 15 and 16 and operation charts inFIGS. 17A to 17C and FIGS. 18A and 18B illustrating the conveyance stateof the sheet.

The control unit 1170 determines which mode is set based on the type andgrammage in FIG. 4 among information about the sheet input from theoperation unit. In other words, in step S101, the control unit 1170determines whether the transfer speed is similar to the fixing speed inmode (mode 4) with reference to the table in FIG. 4.

If the mode is not the mode 4 (or the fixing speed is higher than thetransfer speed) (NO in step S101), then in step S102, the control unit1170 determines whether the sheet is of a small-sized based oninformation about size among information about the type of the sheet. Inthe present exemplary embodiment, the control unit 1170 determines thatthe sheet is of a small-sized in a case where the length of the sheet tobe conveyed in the conveyance direction is shorter than the lengthbetween the secondary transfer nip N2 and the upstream end of the secondconveyance belt 21 and shorter than the length between the downstreamend of the first conveyance belt 11 and the fixing nip N. If the sheetis not of a small-sized (NO in step S102), the control unit 1170executes the large-size conveyance control illustrated in FIG. 16. FIGS.17A to 17C and FIGS. 18A and 18B are time-series charts illustrating theoperation of conveyance of the sheet in a case where the transfer speedis higher than the fixing speed and the sheet is of a large-sized(hereinafter referred to as large-size conveyance control).

In step S121, in the large-size conveyance control illustrated in FIG.16, the control unit 1170 controls the heating roller drive motor 54 sothat a fixing speed is kept at the fixing speed VF illustrated in FIG. 4according to the type and grammage of the sheet. In step S121, thecontrol unit 1170 controls the first conveyance belt drive motor 14 sothat the conveyance speed of the first belt conveyance unit 10 becomesequal to 300 mm/sec, which is equal to the transfer speed, as the firstspeed at which the sheet advancing from the transfer nip N2 is received.In step S121, the control unit 1170 controls the second conveyance beltdrive motor 24 so that the conveyance speed of the second beltconveyance unit 20 becomes equal to the fixing speed VF as the secondspeed at which the leading edge of the sheet enters the fixing nip N.The control unit 1170 performs control so that the sheet is suckedagainst the first conveyance belt 11 (ON) in the first belt conveyanceunit 10. More specifically, the control unit 1170 controls the operationof the first shutter drive motor 31 to move the first shutter 17 to theopen position so that the first suction fan 15 sucks the sheet againstthe first conveyance belt 11. On the other hand, the control unit 1170performs control so that the sheet is not sucked against the secondconveyance belt 21 (OFF). In step S121, the control unit 1170 controlsthe operation of the second shutter drive motor 30 to move the secondshutter 27 to the closed position so that the second suction fan 25 doesnot suck the sheet against the second conveyance belt 21.

It is exemplified herein that the first speed of the first beltconveyance unit 10 at which the sheet advancing from the transfer nip N2is received and conveyed is equal to the transfer speed. However, as isthe case with the first exemplary embodiment, the first speed of thefirst belt conveyance unit 10 has only to be speed at which the sheetadvancing from the transfer nip N2 can be smoothly conveyed withoutsignificantly affecting the transfer of the toner image and does notalways need to be equal to the transfer speed.

It is exemplified herein that the second speed of the second beltconveyance unit 20 at which the leading edge of the sheet enters thefixing nip N is equal to the fixing speed. However, as is the case withthe first exemplary embodiment, the second speed of the second beltconveyance unit 20 has only to be speed at which the sheet can beconveyed such that the toner image can be satisfactorily fixed when theleading edge of the sheet enters the fixing nip N and does not alwaysneed to be equal to the fixing speed VF.

In the operation condition set in step S121, the sheet sent out from thesecondary transfer nip N2 is conveyed by the first belt conveyance unit10. At this point, since the circumferential speed of the firstconveyance belt 11 (the conveyance speed of the first belt conveyanceunit 10) is set to the first speed (equal to 300 mm/sec being thetransfer speed in the present exemplary embodiment) at which the sheetadvancing from the transfer nip N2 is received and conveyed and thesheet is sucked against the first conveyance belt 11, the sheet issmoothly conveyed (refer to FIG. 17A).

Then, the sheet is sent to the second belt conveyance unit 20 by thefirst belt conveyance unit 10 and the leading edge of the sheet reachesonto the second belt conveyance unit 20(refer to FIG. 17B). The sheet isnot sucked against the second conveyance belt 21 at this point. Theconveyance force of the first conveyance belt 11 in the first beltconveyance unit 10, which performs the suction operation, is greaterthan that of the second conveyance belt 21 in the second belt conveyanceunit 20. For this reason, the leading edge of the sheet bridging thefirst and second conveyance belts 11 and 21 slides on thecircumferential surface of the second conveyance belt 21, whosecircumferential speed is set to the second speed higher than thetransfer speed, and the sheet itself is conveyed at the first speedbeing the circumferential speed of the first conveyance belt 11.Although the leading edge of the sheet to which the toner image is beingtransferred at the secondary transfer nip N2 contacts the secondconveyance belt 21 rotated at the second speed higher than the firstspeed, the sheet is conveyed at the first speed by the first conveyancebelt 11. Consequently, the toner image is satisfactorily transferred atthe secondary transfer nip N2. Hereinafter, such a state is referred toas first suction state, where the operation of the suction unit iscontrolled to make the conveyance force of the second belt conveyanceunit 20 smaller than that of the first belt conveyance unit 10 and thesheet is conveyed at the circumferential speed of the first conveyancebelt 11 while sliding on the second conveyance belt 21.

In step S122, the control unit 1170 determines whether the leading edgeof the sheet conveyed by the first and second belt conveyance units 10and 20 is detected by the sheet leading edge detection sensor 26 at thedetection position P. If the leading edge of the sheet conveyed by thefirst and second belt conveyance units 10 and 20 is detected by thesheet leading edge detection sensor 26 (YES in step S122), theprocessing proceeds to step S123. In step S123, the control unit 1170controls the operation of the first shutter drive motor 31 to move thefirst shutter 17 to the closed position so that the sheet is not suckedagainst the first conveyance belt 11. Furthermore, the control unit 1170controls the operation of the second shutter drive motor 30 to move thesecond shutter 27 to the open position so that the sheet is suckedagainst the second conveyance belt 21.

In this state, the sheet is not sucked against the first conveyance belt11, but the sheet is sucked against the second conveyance belt 21 (asecond suction state). Therefore, the conveyance force of the secondconveyance belt 21 in the second belt conveyance unit 20, which performsthe suction operation, is greater than that of the first conveyance belt11 in the first belt conveyance unit 10. FIG. 17C illustrates that theleading edge of the sheet conveyed by the first and the second beltconveyance units 10 and 20 reaches the sheet leading edge detectionsensor 26 and the operation state of suction of the sheet is completed.

The detection position P is set to such a position that the trailingedge of the sheet already passes the secondary transfer nip N2 when theleading edge of the sheet reaches the detection position P. For thisreason, the trailing edge of the sheet already passes the secondarytransfer nip N2 when the operation state of suction of the sheet ischanged from the first suction state to the second suction state. Thechange of the suction state to the second suction state does not affectthe transfer of the toner image to the sheet. The detection position Pis set to such a position that the first and second shutters 17 and 27can be moved to the close and open positions respectively.

The sheet is conveyed as described below in a state where the sheet isnot sucked against the first conveyance belt 11, but the sheet is suckedagainst the second conveyance belt 21. In other words, the conveyanceforce of the second conveyance belt 21 which performs the suctionoperation is greater than that of the first conveyance belt 11 whichdoes not perform the suction operation. The sheet lying on the first andsecond conveyance belts 11 and 21 is pulled and conveyed by the secondconveyance belt 21 whose circumferential speed is set to the secondspeed higher than that of the first conveyance belt 11 while sliding onthe outer circumference of the first conveyance belt 11 at the secondspeed (the fixing speed in the present exemplary embodiment) at whichthe leading edge of the sheet enters the fixing nip N. The sheet ispulled by the second conveyance belt 21 at the second speed whilesliding on the outer circumference of the first conveyance belt 11 andthe leading edge reaches the fixing nip N.

When the leading edge of the sheet S1 conveyed by the second conveyancebelt 21 enters the fixing nip N, the sheet is conveyed at the secondspeed at which the leading edge of the sheet S1 is caused to enter thefixing nip N by the second conveyance belt 21. A great differencebetween the speed of the sheet at the time when the leading edge of thesheet S1 enters the fixing nip N and the fixing speed at the fixing nipN causes a defective fixing. The sheet pulled and conveyed by the secondconveyance belt 21 while sliding on the first conveyance belt 11 isconveyed at the second speed (the fixing speed VF) at which the leadingedge of the sheet is caused to enter the fixing nip N, which does notcause defective in fixing the toner image.

In step S124, the control unit 1170 determines whether the leading edgeof the sheet reaches the fixing nip N. If the leading edge of the sheetreaches the fixing nip N (YES in step S124), then in step S125, thecontrol unit 1170 controls the operation of the first shutter drivemotor 31 to move the first shutter 17 to the open position so that thesheet is sucked against the first conveyance belt 11. In the presentexemplary embodiment, the control unit 1170 determines whether theleading edge of the sheet reaches the fixing nip N, as described below.Specifically, the control unit 1170 receives a signal indicating thatthe leading edge of the sheet is detected from the sheet leading edgedetection sensor 26. The control unit 1170 receives the signal from thesheet leading edge detection sensor 26 and the timer 666 measureswhether a predetermined time interval required by the leading edge ofthe sheet reaching the fixing nip N passes. Thus, the control unit 1170determines whether the leading edge of the sheet reaches the fixing nipN.

The leading edge of the sheet is nipped by the fixing nip N and then thesheet nipped by the fixing nip N is sucked against the first conveyancebelt 11 rotated at the first speed lower than the fixing speed (refer toFIG. 18A). Because a nipping pressure is high at the fixing nip N, theconveyance force of the fixing unit 50 is greater than that of the firstconveyance belt 11. Even if the fixing nip N is subjected to a backtension from the first conveyance belt 11 rotated at the first speedlower than the fixing speed, the fixing unit 50 can stably convey thesheet at the fixing speed VF, so that a defective fixing is not caused.

The reason the sheet is sucked against the first conveyance belt 11 in astate where the leading edge of the sheet 51 is nipped by the fixing nipN and the trailing edge of the sheet 51 lies on the second conveyancebelt 21 is described below. The reason is that the leading edge of thesucceeding sheet S2 can be caused to reach the first conveyance belt 11while the preceding sheet is being conveyed by the fixing nip N (referto FIG. 18A). In other words, the succeeding sheet S2 nipped by thesecondary transfer nip N2 is conveyed by the first conveyance belt 11which performs the suction of the sheet and is set to the first speed atwhich the sheet advancing from the transfer nip N2 is received andconveyed, so that the succeeding sheet S2 can be stably conveyed and adefective transfer is not caused. Thereby, sheets can be conveyed withthe distance between continuous sheets shortened to provide the imageforming apparatus high in productivity.

In step S126, the control unit 1170 determines whether the leading edgeof the succeeding sheet S2 reaches the upstream detection position P1based on the signal from the upstream sheet leading edge detectionsensor 16. If the leading edge of the succeeding sheet S2 reaches theupstream detection position P1 (YES in step S126), then in step S127,the control unit 1170 controls the operation of the second shutter drivemotor 30 to move the second shutter 27 to the closed position so thatthe sheet is not sucked against the second conveyance belt 21.

The reason the sheet is not sucked against the second conveyance belt 21in a state where the leading edge of the preceding sheet is nipped bythe fixing nip N and a part of the preceding sheet lies on the secondconveyance belt 21 (the first suction state) is that an auxiliarytransfer is sufficient for the function required by the secondconveyance belt 21 after the leading edge of the sheet is nipped by thefixing nip N. In other words, the nipping pressure is high in the fixingnip N and the sheet is nipped by the fixing nip N and conveyed at thefixing speed VF, so that the defective fixing is not caused.

Thus, the reason the sheet is not sucked against the second conveyancebelt 21 in a state where the trailing edge of the sheet nipped by thefixing nip N lies on the second conveyance belt 21 is that the leadingedge of the succeeding sheet S2 can be caused to reach the secondconveyance belt 21 while the preceding sheet S1 is being conveyed by thefixing nip N. Thereby, sheets can be conveyed with the distance betweencontinuous sheets shortened to provide the image forming apparatus highin productivity. Although the trailing edge of the succeeding sheet S2is on the secondary transfer nip N2 and the leading edge of thesucceeding sheet S2 lies on the second conveyance belt 21, the sheet isnot sucked against the second conveyance belt 21 to allow the succeedingsheet S2 to be stably conveyed by the first conveyance belt 11,preventing the defective transfer from being caused.

In step S128, the control unit 1170 determines whether the job is ended.If the job is not ended (NO in step S128), the processing returns tostep S122. If the job is ended (YES in step S128), the processingreturns to step S103 in FIG. 15 and ends.

Thus, the decrease of the length of the first and second conveyancebelts 11 and 21 in the conveyance direction can provide a downsizedapparatus. Even if the fixing speed VF is higher than the transfer speedVT, the conveyance of the sheet with the state of suction of the sheetagainst the first and second conveyance belts 11 and 21 changed realizesa stable conveyance of the sheet. The change of the state of suction ofthe sheet against the first and second conveyance belts 11 and 21enables the sheet to be conveyed with the distance between the trailingedge of the preceding sheet S1 and the leading edge of the succeedingsheet S2 reduced.

In FIG. 15, if the control unit 1170 determines that the mode is in themode 4 (YES in step S101), i.e., if the control unit 1170 determinesthat the fixing speed is equal to the transfer speed, the processingproceeds to step S104. In step S104, the control unit 1170 controls thefirst and second shutter drive motors 31 and 30 so that the suctionoperation of the sheet is performed in the first and second beltconveyance units 10 and 20. The control unit 1170 controls the firstconveyance belt drive motor 14 so that the conveyance speed of the firstbelt conveyance unit 10 becomes equal to the first speed (300 mm/secequal to the transfer speed, in the present exemplary embodiment). Thecontrol unit 1170 controls the second conveyance belt drive motor 24 sothat the conveyance speed of the second belt conveyance unit 20 becomesequal to the first speed (300 mm/sec equal to the transfer speed, in thepresent exemplary embodiment). The sheet is continuously conveyed as itis. At this point, the conveyance state is similar to the one in thefirst exemplary embodiment illustrated in FIGS. 10A and 10B, so that thedescription thereof is omitted herein.

If the mode is not the mode 4 (i.e., if the fixing speed is higher thanthe transfer speed) (NO in step S101) and the sheet is of a small-sized(YES in step S102), the processing proceeds to step S105. In step S105,the control unit 1170 controls the first and second shutter drive motors31 and 30 so that the suction operation of the sheet is performed in thefirst and second belt conveyance units 10 and 20. The control unit 1170controls the first conveyance belt drive motor 14 so that the conveyancespeed of the first belt conveyance unit 10 becomes equal to the firstspeed (300 mm/sec equal to the transfer speed). The control unit 1170controls the second conveyance belt drive motor 24 so that theconveyance speed of the second belt conveyance unit 20 becomes equal tothe second speed (equal to the fixing speed VF). The sheet iscontinuously conveyed as it is. At this point, the conveyance state issimilar to the one in the first exemplary embodiment illustrated inFIGS. 9A to 9D, so that the description thereof is omitted herein.

In the above exemplary embodiments, the first shutter 17 lying in theclosed position on the first belt conveyance unit 10 substantiallycompletely covers the opening between the first suction fan 15 and thefirst conveyance belt 11. The second shutter 27 lying in the closedposition substantially completely covers the opening. However, in thelarge-size conveyance control, while the toner image is beingtransferred to the sheet, the first and second suction devices have onlyto be operated so that the sheet slides on the second conveyance belt 21by making greater the conveyance force of the first conveyance belt 11than that of the second conveyance belt 21. When the transfer of thetoner image to the sheet is completed and the sheet is caused to enterthe fixing nip N, the first and second suction devices have only to beoperated so that the sheet slides on the first conveyance belt 11 bymaking greater the conveyance force of the second conveyance belt 21than that of the first conveyance belt 11. For example, the firstshutter 17 may half cover the opening in the closed position. The secondshutter 27 of the second belt conveyance unit 20 may half cover theopening in the closed position.

In the second exemplary embodiment, it is exemplified that the state ofsuction of the sheet is changed using the shutter in the first andsecond belt conveyance units 10 and 20. However, the state of suction ofthe sheet may be changed by controlling the rotation of the first andsecond suction fans 15 and 25 in the first and second belt conveyanceunits 10 and 20. In the large-size conveyance control, while the tonerimage is being transferred to the sheet, for example, the first suctionfan 15 is rotated at top speed and the second suction fan 25 is stopped(or rotated at half-top speed). When the transfer of the toner image tothe sheet is completed and the sheet is caused to enter the fixing nipN, the second suction fan 25 is rotated at top speed and the firstsuction fan 15 is stopped or rotated at half-top speed.

In the large-size conveyance control, the control unit 1170 determinesthe timing at which the first suction state is changed to the secondsuction state in the first and second belt conveyance units 10 and 20depending on whether the leading edge of the sheet is detected by thesheet leading edge detection sensor 26. However, the timing at which thefirst suction state is changed to the second suction state has only tobe fixed after the trailing edge of the sheet to be conveyed passes thesecondary transfer nip N2, so that the present invention is not limitedto the above configuration. For example, a sensor for detecting thesheet is provided near the registration roller 115. A signal is receivedfrom the sensor and then the timer 666 measures whether the timerequired for the trailing edge of the sheet passing the secondarytransfer nip N2 elapses. Thus, the control unit 170 determines that thetrailing edge of the sheet passes the secondary transfer nip N2.Alternatively, a sensor for detecting that the trailing edge of thesheet passes the transfer nip N2 may be provided between the transfernip N2 and the first conveyance belt 11 and the suction state may bechanged from the first suction state to the second suction state basedon the signal of the sensor.

Similarly, in the large-size conveyance control, although it isexemplified that the suction operation in the second belt conveyanceunit 20 is stopped and the suction state is returned to the firstsuction state at the timing at which the leading edge of the succeedingsheet is detected by the upstream leading edge detection sensor 16, thepresent invention is not limited to the above configuration. The timingat which the suction state of the first and second belt conveyance units10 and 20 is returned to the first suction state may be earlier than thetiming at which the succeeding sheet S2 reaches the second conveyancebelt 21. For example, a sensor for detecting the sheet is provided nearthe registration roller 115, the suction operation in the second beltconveyance unit 20 is stopped and the suction state may be returned tothe first suction state at the timing earlier than the timing at whichthe succeeding sheet S2 reaches the second conveyance belt 21 based on asignal from the sensor.

The second exemplary embodiment described above achieves the followingeffects.

(1) The fixing speed VF is changed according to the type and grammage ofthe sheet S. For this reason, the fixing unit can provide the sheet withan appropriate quantity of heat according to the type of the sheet Swith the temperature of the fixing unit kept at a constant regulatedtemperature.

(2) The first and second belt conveyance units 10 and 20 arranged in theconveyance direction are provided between the secondary transfer nip N2and the fixing nip N. The conveyance speed V1 of the first beltconveyance unit 10 is taken as the first speed at which the sheetadvancing from the transfer nip N2 is received. The conveyance speed V2of the second belt conveyance unit 20 is changed according to the fixingspeed VF. Therefore, the sheet can be conveyed with the transfer speedVT kept constant irrespective of the setting value of the fixing speedVF which is changed according to the type and grammage of the sheet.

(3) Even if the large-sized sheet is conveyed in a mode in which thefixing speed VF is higher than the transfer speed, the sheet can bestably conveyed with a high productivity using the conveyance belt shortin length in the conveyance direction.

More specifically, while the sheet to which the toner image istransferred at the secondary transfer nip N2 is being conveyed, thesheet is caused to slide on the second conveyance belt 21 by makinggreater the conveyance force of the first conveyance belt 11 than thatof the second conveyance belt 21 (the first suction state). Accordingly,the sheet S which bridges the first and second conveyance belts 11 and21 and to which the toner image is being transferred can be conveyed atthe first speed, so that the toner image can be satisfactorilytransferred to the sheet.

The conveyance force of the second conveyance belt 21 is made greaterthan the first conveyance belt 11 after the trailing edge of the sheetpasses the secondary transfer nip N2 to slide the sheet on the firstconveyance belt 11 (the second suction state). The conveyance speed ofthe sheet S which bridges the first and second conveyance belts 11 and21 to which the transfer of the toner image is finished can be taken asthe second speed. The leading edge of the sheet S which bridges thefirst and second conveyance belts 11 and 21 can be caused to reach thefixing nip N while being conveyed at the second speed.

Thus, the apparatus can be downsized by making shorter the lengthbetween the transfer nip N2 and the upstream end of the secondconveyance belt 21 and between the downstream end of the firstconveyance belt 11 and the fixing nip N than that of the maximum sheet.The apparatus can be downsized, and at the same time, the sheet can beconveyed at the first speed while the toner image is being transferredand stably conveyed at the high second speed after the transfer isfinished.

The operation of the first suction device is controlled so that thesheet is sucked against the first conveyance belt 11 in a state wherethe tailing edge of the sheet still lies on the first conveyance belt 11after the leading edge of the preceding sheet reaches the fixing nip N(refer to FIG. 18A). This control allows the first conveyance belt 11 tostably transfer the succeeding sheet sent from the secondary transfernip N2 at the first speed even in a state where the trailing edge of thepreceding sheet conveyed by the fixing nip N lies on the firstconveyance belt 11. Consequently, the toner image is satisfactorilytransferred to the succeeding sheet, and at the same time, the distancebetween the continuously conveyed sheets is decreased to allow theimprovement of productivity.

The control unit 170 performs control so that the suction state ischanged to the first suction state in a state where the tailing edge ofthe preceding sheet still lies on the second conveyance belt 21 afterthe leading edge of the preceding sheet reaches the fixing nip N andbefore the succeeding sheet S2 reaches the second conveyance belt 21(refer to FIG. 18B). In other words, their respective suction states arechanged so that the conveyance force of the first belt conveyance unit10 can be made greater than that of the second belt conveyance unit 20.This control allows the stable conveyance of the succeeding sheet sentfrom the secondary transfer nip N2 at the first speed with thesucceeding sheet bridging the first and second conveyance belts 11 and21 even in a state where the trailing edge of the preceding sheet lieson the second conveyance belt 21. Consequently, the toner image issatisfactorily transferred to the succeeding sheet, and at the sametime, the distance between the continuously conveyed sheets is decreasedto allow the improvement of productivity.

As described above, in the image forming apparatus of the presentexemplary embodiment, the apparatus is small, and at the same time, thesheet can be smoothly conveyed with the distance between the tailingedge of the preceding sheet and the leading edge of the succeeding sheetbeing small even if the fixing speed VF is higher than the transferspeed VT.

(4) When the sheet to which the toner image has been already transferredis conveyed with the sheet bridging the first and second conveyancebelts 11 and 21, the sheet is conveyed while being pulled by the secondconveyance belt at a speed higher than the transfer speed so that thetrailing edge of the sheet slides on the first conveyance belt 11.Therefore, the speed of the sheet is changed to a high speed earlierthan the timing at which the trailing edge of the sheet leaves thedownstream end of the first conveyance belt 11. Accordingly, the timerequired for the sheet advancing from the transfer nip N2 reaching thefixing nip N is short to improve productivity.

(5) The distance between the secondary transfer nip N2 and the fixingnip N is longer than the length of a sheet which is conveyable andmaximum in size. Therefore, as illustrated in FIG. 10B, for example,after the tailing edge of the sheet S1 carrying the toner image yet tobe fixed leaves the secondary transfer nip N2, the leading edge of thesheet S1 reaches the fixing nip N. Even if a shock occurs when thickpaper plunges into the fixing nip N, the tailing edge of the sheet S1leaves the secondary transfer nip N2, so that image defectiveness can beminimized.

(6) The shutter 17 is opened or closed to cause the suction fan 15 ofthe first belt conveyance unit 10 to suck the sheet or not and theshutter 27 is also used in the second belt conveyance unit 20. Thisconfiguration further achieves the following effect. The suction statecan be changed by the suction fan with a good response.

The configuration is inexpensive in which the rotation operation of thefirst and second suction fans is controlled without the use of theshutters 17 and 27 to change the suction state where the sheet is suckedagainst the first and second conveyance belts 11 and 21 because otherdevices do not need to be added for the change of the suction state.

Although the first and second exemplary embodiments describe an exampleof the image forming apparatus for forming a full-color toner image on asheet, the exemplary embodiments can be applied to an image formingapparatus for forming also a monochrome toner image thereon.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures, and functions.

This application claims priority from Japanese Patent Applications No.2010-227527 filed Oct. 7, 2010, No. 2010-227528 filed Oct. 7, 2010, andNo. 2010-227529 filed Oct. 7, 2010, which are hereby incorporated byreference herein in their entirety.

What is claimed is:
 1. An image forming apparatus comprising: a transfernip configured to transfer a toner image to a sheet conveyed at atransfer speed; a first conveyance belt configured to convey the sheetto which the toner image is transferred at the transfer nip; a secondconveyance belt provided downstream of the first conveyance belt in aconveyance direction in which the sheet is conveyed and configured toconvey the sheet; a fixing nip provided downstream of the secondconveyance belt in the conveyance direction and configured to fix thetoner image to the sheet while conveying the sheet; a fixing drive unitconfigured to drive the fixing nip so that the fixing nip conveys thesheet at a speed that is higher than the transfer speed in a high speedfixing mode in which the fixing nip fixes the toner image to the sheetwhile conveying the sheet at the speed that is higher than the transferspeed; and a control unit configured to perform control so that acircumferential speed of the second conveyance belt is changed from afirst speed at which the sheet advances from the transfer nip to asecond speed at which a leading edge of the sheet enters the fixing nipand which is higher than the first speed after a trailing edge of thesheet, conveyed by the first conveyance belt whose circumferential speedis set to the first speed and the second conveyance belt whosecircumferential speed is set to the first speed, passes the transfer nipand before the leading edge of the sheet reaches the fixing nip in thehigh speed fixing mode and in response to a length of the conveyed sheetin the conveyance direction being longer than a length from the transfernip to an upstream end of the second conveyance belt, wherein, after thecircumferential speed of the second conveyance belt is changed to thesecond speed, the sheet sucked against the second conveyance belt isconveyed by the second conveyance belt while sliding on the firstconveyance belt.
 2. The image forming apparatus according to claim 1,wherein, in the high speed fixing mode, the leading edge of a succeedingsheet advancing from the transfer nip in succession to a preceding sheetreaches the first conveyance belt with the preceding sheet being on thefirst and second conveyance belts.
 3. The image forming apparatusaccording to claim 2, wherein, in the high speed fixing mode, thecontrol unit changes the circumferential speed of the second conveyancebelt to the first speed after a leading edge of the preceding sheetconveyed by the second conveyance belt reaches the fixing nip, when thepreceding sheet lies on the second conveyance belt, and before thesucceeding sheet reaches the second conveyance belt.
 4. The imageforming apparatus according to claim 1, further comprising: a firstsuction device configured to suck the sheet against a circumferentialsurface of the first conveyance belt; and a second suction deviceconfigured to suck the sheet against a circumferential surface of thesecond conveyance belt, wherein a force by which the second suctiondevice sucks the sheet against the circumferential surface of the secondconveyance belt is made greater than a force by which the first suctiondevice sucks the sheet against the circumferential surface of the firstconveyance belt to cause the second conveyance belt to pull and conveythe sheet so that the sheet slides on the first conveyance belt when theleading edge of the sheet reaches the fixing nip.
 5. The image formingapparatus according to claim 1, further comprising: a first suctiondevice configured to suck the sheet against a circumferential surface ofthe first conveyance belt; and a second suction device configured tosuck the sheet against a circumferential surface of the secondconveyance belt, wherein a friction coefficient of the circumferentialsurface of the second conveyance belt is made greater than a frictioncoefficient of the circumferential surface of the first conveyance beltto cause the second conveyance belt against which the sheet is sucked bythe second suction device to pull and convey the sheet so that the sheetslides on the first conveyance belt against which the sheet is sucked bythe first suction device when the leading edge of the sheet reaches thefixing nip.
 6. The image forming apparatus according to claim 1,wherein, in response to a length of the conveyed sheet in the conveyancedirection being shorter than a length from the transfer nip to anupstream end of the second conveyance belt and shorter than a lengthfrom a downstream end of the first conveyance belt to the fixing nip,the sheet advancing from the transfer nip to the fixing nip is conveyedwith the circumferential speed of the first conveyance belt set to thefirst speed and the circumferential speed of the second conveyance beltset to the second speed and the sheet lying on the first and secondconveyance belts is pulled and conveyed by the second conveyance beltwhile sliding on the first conveyance belt at a speed that is higherthan the first speed.
 7. The image forming apparatus according to claim1, wherein, in the high speed fixing mode and in response to a length ofthe conveyed sheet in the conveyance direction being longer than alength from the transfer nip to an upstream end of the second conveyancebelt and longer than a length from a downstream end of the firstconveyance belt to the fixing nip, the control unit performs control sothat the circumferential speed of the second conveyance belt is changedto the second speed after a trailing edge of the sheet, conveyed by thefirst conveyance belt whose circumferential speed is set to the firstspeed and the second conveyance belt whose circumferential speed is setto the first speed, passes the transfer nip and before the leading edgeof the sheet reaches the fixing nip, and, after the circumferentialspeed of the second conveyance belt is changed to the second speed, theleading edge of the sheet reaches the fixing nip with the sheet suckedagainst the second conveyance belt pulled and conveyed by the secondconveyance belt while sliding on the first conveyance belt.
 8. An imageforming apparatus comprising: a transfer nip configured to transfer atoner image to a sheet conveyed at a transfer speed; a first conveyancebelt configured to convey the sheet to which the toner image istransferred at the transfer nip; a second conveyance belt provideddownstream of the first conveyance belt in a conveyance direction inwhich the sheet is conveyed and configured to convey the sheet; a fixingnip provided downstream of the second conveyance belt in the conveyancedirection and configured to fix the toner image to the sheet whileconveying the sheet; a drive unit configured to drive the fixing nip sothat the fixing nip conveys the sheet at a speed that is higher than thetransfer speed in a high speed fixing mode in which the fixing nip fixesthe toner image to the sheet while conveying the sheet at the speed thatis higher than the transfer speed; a suction unit provided for each ofthe first and second conveyance belts and configured to suck the sheet;and a control unit configured to control an operation of the suctionunit for sucking the sheet against each of the first and secondconveyance belts, wherein, in the high speed fixing mode and in responseto a length of the conveyed sheet in the conveyance direction beinglonger than a length from the transfer nip to an upstream end of thesecond conveyance belt, a circumferential speed of the first conveyancebelt is set to a first speed at which the sheet advances from thetransfer nip and a circumferential speed of the second conveyance beltis set to a second speed at which a leading edge of the sheet enters thefixing nip and which is higher than the first speed and the control unitcontrols the suction unit so that the sheet is sucked against the firstconveyance belt and slides on the second conveyance belt in a state thatthe sheet to which the toner image is transferred by the transfer nip isbeing conveyed by the first conveyance belt and controls the suctionunit to cause the second conveyance belt to pull and convey the sheet sothat the sheet slides on the first conveyance belt after a trailing edgeof the sheet, conveyed by the first conveyance belt whosecircumferential speed is set to the first speed, passes the transfernip.
 9. The image forming apparatus according to claim 8, wherein, inthe high speed fixing mode, the control unit controls the suction unitso that the sheet is sucked against the first conveyance belt and thesheet is not sucked against the second conveyance belt until a trailingedge of the conveyed sheet passes the transfer nip, and controls thesuction unit so that the sheet is sucked against the second conveyancebelt and the sheet is not sucked against the first conveyance belt afterthe trailing edge of the conveyed sheet passes the transfer nip andbefore the leading edge of the sheet reaches the fixing nip.
 10. Theimage forming apparatus according to claim 9, wherein the control unitcontrols the operation of the suction unit so that the sheet is notsucked against the second conveyance belt when the leading edge of apreceding sheet reaches the fixing nip and lies on the second conveyancebelt and a succeeding sheet conveyed in succession to the precedingsheet is conveyed onto the second conveyance belt by the firstconveyance belt while the preceding sheet is conveyed by the fixing nipand after the preceding sheet is not sucked against the secondconveyance belt.
 11. The image forming apparatus according to claim 8,wherein the suction unit includes a first suction device configured tosuck the sheet against the first conveyance belt and a second suctiondevice configured to suck the sheet against the second conveyance belt,wherein the first suction device includes a first fan and a firstshutter configured to open and close between the first fan and the firstconveyance belt, wherein the second suction device includes a second fanand a second shutter configured to open and close between the second fanand the second conveyance belt, and wherein the control unit opens andcloses the first and second shutters to control the operation of thesuction unit for sucking the sheet against the first and secondconveyance belts.
 12. The image forming apparatus according to claim 8,wherein, in the high speed fixing mode and in response to a length ofthe conveyed sheet in the conveyance direction being longer than alength from the transfer nip to an upstream end of the second conveyancebelt and longer than a length from a downstream end of the firstconveyance belt to the fixing nip, the circumferential speed of thefirst conveyance belt is set to the first speed and the circumferentialspeed of the second conveyance belt is set to the second speed and thecontrol unit controls the suction unit so that the sheet is suckedagainst the first conveyance belt and slides on the second conveyancebelt in a state that the sheet to which the toner image is transferredby the transfer nip is being conveyed by the first conveyance belt andcontrols the suction unit to cause the second conveyance belt to pulland convey the sheet so that the sheet slides on the first conveyancebelt when the leading edge of the sheet reaches the fixing nip.
 13. Animage forming apparatus comprising: a transfer nip configured totransfer a toner image to a sheet conveyed at a transfer speed; a firstconveyance belt configured to convey the sheet to which the toner imageis transferred at the transfer nip; a second conveyance belt provideddownstream of the first conveyance belt in a conveyance direction inwhich the sheet is conveyed and configured to convey the sheet; a fixingnip provided downstream of the second conveyance belt in the conveyancedirection and configured to fix the toner image to the sheet whileconveying the sheet; and a control unit configured to perform control sothat a circumferential speed of the second conveyance belt is changedfrom a first speed at which the sheet advances from the transfer nip toa second speed at which a leading edge of the sheet enters the fixingnip and which is higher than the first speed after a trailing edge ofthe sheet, conveyed by the first conveyance belt whose circumferentialspeed is set to the first speed and the second conveyance belt whosecircumferential speed is set to the first speed, passes the transfer nipand before the leading edge of the sheet reaches the fixing nip inresponse to a length of the conveyed sheet in the conveyance directionbeing longer than a length from the transfer nip to an upstream end ofthe second conveyance belt, wherein, after the circumferential speed ofthe second conveyance belt is changed to the second speed, the sheetsucked against the second conveyance belt is conveyed by the secondconveyance belt while sliding on the first conveyance belt.
 14. Theimage forming apparatus according to claim 13, wherein the leading edgeof a succeeding sheet advancing from the transfer nip in succession to apreceding sheet reaches the first conveyance belt with the precedingsheet being on the first and second conveyance belts.
 15. The imageforming apparatus according to claim 13, further comprising: a firstsuction device configured to suck the sheet against a circumferentialsurface of the first conveyance belt; and a second suction deviceconfigured to suck the sheet against a circumferential surface of thesecond conveyance belt, wherein a force by which the second suctiondevice sucks the sheet against the circumferential surface of the secondconveyance belt is made greater than a force by which the first suctiondevice sucks the sheet against the circumferential surface of the firstconveyance belt to cause the second conveyance belt to pull and conveythe sheet so that the sheet slides on the first conveyance belt when theleading edge of the sheet reaches the fixing nip.
 16. The image formingapparatus according to claim 13, further comprising: a first suctiondevice configured to suck the sheet against a circumferential surface ofthe first conveyance belt; and a second suction device configured tosuck the sheet against a circumferential surface of the secondconveyance belt, wherein a friction coefficient of the circumferentialsurface of the second conveyance belt is made greater than a frictioncoefficient of the circumferential surface of the first conveyance beltto cause the second conveyance belt against which the sheet is sucked bythe second suction device to pull and convey the sheet so that the sheetslides on the first conveyance belt against which the sheet is sucked bythe first suction device when the leading edge of the sheet reaches thefixing nip.
 17. The image forming apparatus according to claim 13,wherein, in response to a length of the conveyed sheet in the conveyancedirection being shorter than a length from the transfer nip to anupstream end of the second conveyance belt and shorter than a lengthfrom a downstream end of the first conveyance belt to the fixing nip,the sheet advancing from the transfer nip to the fixing nip is conveyedwith the circumferential speed of the first conveyance belt set to thefirst speed and the circumferential speed of the second conveyance beltset to the second speed and the sheet lying on the first and secondconveyance belts is pulled and conveyed by the second conveyance beltwhile sliding on the first conveyance belt at a speed that is higherthan the first speed.
 18. The image forming apparatus according to claim13, wherein, in response to a length of the conveyed sheet in theconveyance direction being longer than a length from the transfer nip toan upstream end of the second conveyance belt and longer than a lengthfrom a downstream end of the first conveyance belt to the fixing nip,the control unit performs control so that the circumferential speed ofthe second conveyance belt is changed to the second speed after atrailing edge of the sheet, conveyed by the first conveyance belt whosecircumferential speed is set to the first speed and the secondconveyance belt whose circumferential speed is set to the first speed,passes the transfer nip and before the leading edge of the sheet reachesthe fixing nip, and, after the circumferential speed of the secondconveyance belt is changed to the second speed, the leading edge of thesheet reaches the fixing nip with the sheet sucked against the secondconveyance belt pulled and conveyed by the second conveyance belt whilesliding on the first conveyance belt.
 19. An image forming apparatuscomprising: a transfer nip configured to transfer a toner image to asheet conveyed at a transfer speed; a first conveyance belt configuredto convey the sheet to which the toner image is transferred at thetransfer nip; a second conveyance belt provided downstream of the firstconveyance belt in a conveyance direction in which the sheet is conveyedand configured to convey the sheet; a fixing nip provided downstream ofthe second conveyance belt in the conveyance direction and configured tofix the toner image to the sheet while conveying the sheet; a suctionunit provided for each of the first and second conveyance belts andconfigured to suck the sheet; and a control unit configured to controlan operation of the suction unit for sucking the sheet against each ofthe first and second conveyance belts, wherein, in response to a lengthof the conveyed sheet in the conveyance direction being longer than alength from the transfer nip to an upstream end of the second conveyancebelt, a circumferential speed of the first conveyance belt is set to afirst speed at which the sheet advances from the transfer nip and acircumferential speed of the second conveyance belt is set to a secondspeed at which a leading edge of the sheet enters the fixing nip andwhich is higher than the first speed and the control unit controls thesuction unit so that the sheet is sucked against the first conveyancebelt and slides on the second conveyance belt in a state that the sheetto which the toner image is transferred by the transfer nip is beingconveyed by the first conveyance belt and controls the suction unit tocause the second conveyance belt to pull and convey the sheet so thatthe sheet slides on the first conveyance belt after a trailing edge ofthe sheet, conveyed by the first conveyance belt whose circumferentialspeed is set to the first speed, passes the transfer nip.
 20. The imageforming apparatus according to claim 19, wherein the control unitcontrols the suction unit so that the sheet is sucked against the firstconveyance belt and the sheet is not sucked against the secondconveyance belt until a trailing edge of the conveyed sheet passes thetransfer nip, and controls the suction unit so that the sheet is suckedagainst the second conveyance belt and the sheet is not sucked againstthe first conveyance belt after the trailing edge of the conveyed sheetpasses the transfer nip and before the leading edge of the sheet reachesthe fixing nip.
 21. The image forming apparatus according to claim 19,wherein the suction unit includes a first suction device configured tosuck the sheet against the first conveyance belt and a second suctiondevice configured to suck the sheet against the second conveyance belt,wherein the first suction device includes a first fan and a firstshutter configured to open and close between the first fan and the firstconveyance belt, wherein the second suction device includes a second fanand a second shutter configured to open and close between the second fanand the second conveyance belt, and wherein the control unit opens andcloses the first and second shutters to control the operation of thesuction unit for sucking the sheet against the first and secondconveyance belts.
 22. The image forming apparatus according to claim 19,wherein, in response to a length of the conveyed sheet in the conveyancedirection being longer than a length from the transfer nip to anupstream end of the second conveyance belt and longer than a length froma downstream end of the first conveyance belt to the fixing nip, thecircumferential speed of the first conveyance belt is set to the firstspeed and the circumferential speed of the second conveyance belt is setto the second speed and the control unit controls the suction unit sothat the sheet is sucked against the first conveyance belt and slides onthe second conveyance belt in a state that the sheet to which the tonerimage is transferred by the transfer nip is being conveyed by the firstconveyance belt and controls the suction unit to cause the secondconveyance belt to pull and convey the sheet so that the sheet slides onthe first conveyance belt when the leading edge of the sheet reaches thefixing nip.